Did your baby never suffer from colic then newtohere? Mine did even whilst being breastfed. It's not as simple as lifting up your top and going back to sleep. Breastfeeding hurt me with both of mine, for some women even though they are doing it right, it just hurts. I was in pain everytime I fed for the first 6 months.
I gave up feeding dd because of it after 4 months and I never regretted it.
Also they cry for things other than milk, they have a tendency to soil their nappies at 2am and 4am on the dot. They cry because they are bored, because they have colic, because they feel unwell. It's mentally and physically exhausting caring for a baby, entire days just blur and you don't know if it's lunchtime or suppertime or when the last time you ate was. All you can think about is sleep for those first few months.
Postpartum psychosis is NOT PND. Some women hallucinate, some hear voices (I did, although mine was whilst still pg), it can be a very scary time and you may not be able to care for yourself properly. Breastfeeding may work for you, but then again it may not, it is not a magical solution for staving off PND or any other kind of depression, if it was then believe me, the NCT and LLL would be announcing it from the rooftops.
We are advising the OP to look into it by all means, to question the psychiatrists advice (and I second that he won't know shit about breastfeeding) and to get a second opinion. But the OP has tried coming off the meds once and it didn't work. It would be dangerous to suggest she try again because breastfeeding will sort it all out.
Go easy ellymouse. Your baby needs your love and attention more than your milk. I was bottlefed too and am absolutely fine. My dh's nephew was exclusively breastfed by his organic vegetable eating mother and he has severe asthma. Bottle milk now is better than it was in the 70s, 80s or even 90s. Whilst it's no substitute, it's not evil either. And if it means that you get to enjoy your baby rather than going around in a constant state of anxiety then I would choose the bottle.
But if you can compromise, ask for a different type of med or a lower dosage or whatever then do so. Only you know how you would cope without your meds, so once you have all the info the choice is yours and yours alone.